A power converter or power supply is a device which converts one form of energy to another, with the two terms being used interchangeably. There are several types of power converters typically used in the semiconductor industry: (1) direct current (dc) to direct current converters; (2) alternating current (ac) to direct current converters; (3) dc to ac converters; and (4) ac to ac converters. The circuit components used to fabricate such devices include inductors, capacitors, resistors, transformers, and switching elements, where the switching elements are usually in the form of an integrated circuit which performs the control functions for the overall device.
FIG. 1 is a representation of a typical dc to dc power converter 10. Converter 10 is connected at the input side to a source 12 which provides a signal to be converted, and at the output side to a load 14. Converter 10 is constructed from the elements described previously; switching elements 16, inductors 18, transformers 20, and passive, discrete elements, such as capacitors 22. Switching elements 16 may take the form of a fast recovery diode, a bipolar junction transistor, a MOSFET, a gate turn-off thyristor, or other similarly functioning components. The power conversion function required in a specific situation will be accomplished by a combination of the above elements, where the switching or control elements are often manufactured in the form of an integrated circuit chip.
FIG. 2 is a schematic of a typical circuit for an ac to dc converter 30. In this circuit, a dc to dc converter 32 is preceded by a rectifier-filter network 34. An ac signal 36 is input to ac to dc converter 30, the signal is transformed to a dc signal by the action of network 34, and the resultant signal is then converted by the action of dc to dc converter 32. A load 38 is connected to the output of dc to dc converter 32. Direct current to alternating current (dc to ac) and alternating current to alternating current (ac to ac) converters typically use ac to dc and dc to dc converters as sub-systems which are combined to obtain the desired output.
The components of a power supply or converter are typically mounted on a printed circuit board or ceramic substrate using surface mounting, through-hole, or chip-on-board technologies known in the industry. As mentioned above, the switching elements may be in the form of an integrated circuit which is mounted separately on the board from the passive elements, inductor(s), and transformer(s), using either the surface mounting or through-hole mounting techniques mentioned. The entire board or substrate is then enclosed in a plastic case or encapsulated in plastic.
FIG. 3 is a cut-away view illustrating the design of a typical prior art dc to dc power converter 100. In this example, a transistor 120 or other heat generating element is surface or through-hole mounted on a printed circuit board 122. Heat generating element 120 may be in thermal contact with a heatsink 124 in order to provide better dissipation of the heat generated by element 120 during operation of power converter 100. Other elements 125 and 126 of the circuitry of power converter 100 are similarly mounted on printed circuit board 122. An encapsulating compound 128 can then be used to enclose the components of power converter 100, although the use of this compound is optional. A case 130 is then used to complete the package. As shown in FIG. 3, a package heatsink 132 may be placed in thermal contact with and attached to converter package 130 to provide added heat dissipation from the components within package 130 to the ambient environment. In such a situation, a thermal compound 134 may be placed between package 130 and heatsink 132 to provide the desired degree of heat conduction between those elements.
A primary heat source of the power converter 100 of FIG. 3 is the IR drop across the collector to emitter potential of transistor 120. The power drop across the junction resistance of transistor 120 is dissipated as heat. In the case of the simple example of FIG. 3, the heat generated by transistor 120 must be conducted through several thermal interfaces prior to being conducted out to the ambient environment. These interfaces include: 1) the collector to emitter junction to the transistor case; 2) the transistor case to the transistor heatsink; 3) the heatsink to the encapsulating compound and/or printed circuit board; 4) the printed circuit board to the encapsulating compound; 5) the encapsulating compound to the package case; and 6) the package case to the thermal compound and package heatsink if one is used. As the generated heat is conducted through each thermal interface, a temperature drop will occur. This reduces the efficiency with which the heat initially generated by the transistor can be removed from the package.
As noted, the electrical power delivered to the power supply or converter results in the internal production of large amounts of heat. The heat generated by the components of the converter must be removed efficiently in order to prevent failure of the device. This is because most all of the common failure mechanisms are enhanced as the result of an increase in temperature.
Heat removal from power supplies and converters is typically achieved by use of a thermally enhanced printed circuit board which contains a metal wafer on one side to conduct the heat away from the components. A printed circuit board made of a composite material having a relatively high heat capacity may also be used, although this solution to the problem may be too expensive for use in a large scale manufacturing environment. One or more heat sinks may also be affixed to the board to assist with heat dissipation. A heat sink may also be attached to the outside of a package containing a power supply or converter to assist with dissipating the heat conducted through the package. Once the heat is transferred to the case or package of a power supply by means of conduction, convective processes transfer the heat to the ambient environment.
Even with these approaches to heat dissipation, presently available power supplies and power converters have several disadvantages. The devices are relatively large in size since all of the components are laid out on a printed circuit board. This can impact the final size of products in which they are incorporated. For instance, such power supply and power converter designs are generally unsuited for use in portable electronic products. Power supply and converter designs which utilize many thermal interfaces between the heat generating components and the ambient environment are often inefficient dissipators of heat. Another issue is that the cost of manufacturing the device can become excessive if a ceramic substrate or thermally enhanced board is used. In addition, the devices lack the high degree of reliability which can be obtained by the use of modem semiconductor packaging techniques. This is because the method of connecting the components to the printed circuit board and the heat dissipation ability of the final product can be sources of device failure which are not present when some modem packaging methods, such as transfer molding, are used.
What is desired is a design for and method of assembling a power supply or power converter which produces a smaller, less expensive, and more reliable device than those presently available. It is also desirable to have a more efficient design for removing heat from the power supply or converter by reducing the number of thermal interfaces between the heat generating components and the ambient environment.